This invention relates to the precision coating of surfaces and more particularly to extrusion coating substrates presenting a non-linear leading edge to provide a uniform, precision, coating.
It is often necessary or desired to provide a coating of a particular substrate. For example, in the micro-electronics industry it is often desired to coat a substrate used in the manufacture of integrated circuits for further processing. Often it is required that such coatings be applied in a very thin coat, such as a thin coat of photoresist used in masking and etching a silicon substrate for manufacturing integrated circuits, which is uniform across the entire surface of the substrate. However, as the coating is so thin, very minute variances in its thickness may not be acceptable.
Accordingly, the prior art has relied upon various methods for providing a continuous, uniform, thin coating of a substrate. However, in the past these methods have been inefficient and, therefore, prone to waste.
For example, a commonly relied upon prior art method of coating a substrate is spin coating. Here a coating material, typically suspended in a solvent based fluid, is deposited in a pool on the substrate to be coated, generally at or near the center of the surface to be coated. Thereafter, the substrate itself is rotated at a high speed about an axis normal to the surface to be coated. Centrifugal forces created by the rotation of the substrate cause the pool of material to migrate toward the edges of the substrate. Accordingly, rotating the substrate for a sufficient length of time at a proper speed will result in a substantially uniform coating having a desired thickness, where a sufficiently ductile coating material is present.
However, the spin method of coating the substrate necessarily results in an amount of coating material being expelled from the surface to be coated. In practice, the expelled portion of coating material may be as great as 90-95% of the material initially deposited in the pool on the substrate. Typically this material which is expelled from the surface is lost as there are often very stringent purity requirements and/or the solvents suspending the material being quick to evaporate making their recycling difficult or impossible. Moreover, spin coating is generally not completely effective in evenly distributing a very viscous coating material.
These coating materials are generally very expensive and therefore the waste that occurs in coating the substrate can be an important consideration. Accordingly, although providing a reliable method for achieving a uniform coating of a substrate, the prior art spin methods introduce an undesired level of waste. Moreover, the amount of waste increases as the size of the substrate increases causing such inefficiencies to be increasingly unpalatable as the industry moves to larger and larger substrates, e.g., 3.5 generation LCD technology and 12 inch silicon wafers.
It should also be appreciated that the excess material discharged from the above mentioned spin technique presents, at a minimum, a requirement for the subsequent handling and cleanup of this substantial amount of unused material. Because of purity of material requirements this discharged material must often be disposed of. However, often times this material, and/or its solvent carrier, are hazardous materials and must be handled with extreme care as well as being disposed of in accordance with stringent guidelines. Likewise, often the solvents utilized in cleanup of such discharged material are hazardous, thus compelling their restricted use.
Additionally, the prior art spin methods of coating the substrate can result in the outer edges and/or the back surface of the substrate also being coated by the material. This can be undesired as subsequent handling of the substrate, having its edges coated, may result in the chipping and peeling of the coating on these edges which may continue to the surface for which a uniform contamination coating is desired. Moreover, coating of these surfaces may also result in the contamination of the surface desired to be coated.
Furthermore, the solvents carrying the desired coating materials in suspension may be highly unstable and, therefore, prone to rapid dissipation, such as through evaporation. Accordingly, uneven coating may result in the aforementioned spin technique where, for example, an appreciable time between depositing the pool of material for spinning, or where the substrate surface to be coated is large.
The above mentioned spin method of coating a substrate, although acceptable for use in coating certain shaped substrates, such as a small circular silicon wafer used in manufacturing integrated circuits, may not provide acceptable results in coating other shaped substrates. For example, spin coating a square or irregularly shaped substrate may result in windage problems when the substrate is spun which cause the coating material to cure in a non-uniform coating. Likewise, because of longer distances between the center of the substrate, where the pool of coating material is deposited prior to spinning, and the outer edges associated with the substrate""s irregular circumference, spinning the coating material onto these substrates may not provide uniform coverage. Similarly, a large silicon wafer may present sufficient distances between the center of the substrate and the outer edges, although presenting a consistent distance, that uniform coverage may be impossible by spin coating alone due to the drying of the coating material as it migrates out along the radius of the substrate being spun. Moreover, due to the fact that the velocity of the outside edge of the large wafer may be relatively large, windage related patterns in the coating may result.
Accordingly, a need exists in the art for a system and method for providing a uniform coating of a desired thickness on a substrate, including substrates of various shapes and sizes, while providing efficient use of a coating material.
A further need exists in the art for the system and method for coating a substrate to minimize coating of surfaces of the substrate which are not desired to be coated, such as a circumferential edge of a surface to be coated.
A still further need exists in the art for the system and method for coating a substrate to provide for simple and efficient cleanup of any excess coating material in order to reduce the time and effort required in cleaning such excess material as well as reduce the amounts of solvents required in such cleanup.
These and other objects, features and technical advantages are achieved by a system and method which utilizes an extrusion or other controlling delivery process to deliver a coating material upon a substrate surface in a uniform coating of a predetermined thickness. After initial deposit of the coating material upon the substrate surface by the extrusion process, the substrate may be spun in order to provide a more uniform coating or a coating of a desired thickness. Alternatively, such as (1) where a coating material is initially deposited in a very viscous state or (2) where a more thin coating is desired than is consistently producible through extrusion alone or for surfaces presenting a high aspect ratio surface to be coated. Spinning of the substrate may be omitted in producing the designed uniform coating according to the present invention.
A preferred embodiment of the present invention utilizes an extrusion die or head providing linear, or substantially linear, extrusion of a coating material at a precisely controlled rate, i.e., the extrusion head moves in a linear motion with respect to the surface to be coated. By selecting the extrusion head presenting an extrusion orifice to be of a sufficient width to correspond to a full width of the substrate to be coated, a single pass of the extrusion head along the substrate, a single pass of the substrate with respect to the extrusion head, or a combination of both, may be utilized to provide complete coating of the substrate. Accordingly, the coating on the substrate may be of a very uniform thickness and/or consistency as no overlap of extruded material or seams between successive co-located extrusion coatings are required to fully coat the surface of the substrate.
Depositing the coating material through extrusion according to the present invention preferably relies upon the use of a coating material in a liquid, or otherwise highly ductile, state, such as a coating material suspended or dissolved in a liquid carrier, which hardens or cures after extrusion, such as by a solvent carrier evaporating. Accordingly, a coating bead may be formed between lips of the extrusion orifice and the substrate surface to be coated, which through capillary action and/or controlled delivery of the liquid coating material to the extrusion head delivers a precisely controllable amount of the material through the extrusion orifice. Therefore, the relative motion of the extrusion head with respect to the substrate surface deposits a desired coating, i.e., wet film, of the material.
It should be appreciated that the coating bead must be precisely controlled if a coating of uniform thickness is to be formed. Accordingly, the relative movement of the extrusion head and the substrate should be carefully controlled to provide for a smooth continuous motion. Additionally, the coating bead should be substantially at a steady state throughout its contact with the substrate surface. Otherwise, the coating may include areas of nonuniform thickness or other inconsistencies in the coating.
The preferred embodiment of the present invention is adapted to provide for the extrusion coating of substrates having a circumferential edge or periphery which does not correspond to the shape of the extrusion head, i.e., utilizing a substantially linear extrusion head with a substrate having a circular, elliptical, or irregular shape. Therefore, relying on a single pass of the extrusion head, having an extrusion orifice width corresponding to the widest portion of the substrate as traversed by the extrusion head, presents areas in which the extrusion orifice does not correlate with the surface of the substrate. This mismatch presents problems in depositing the desired uniform coating. For example, where the substrate is circular and the extrusion head is linear, initially forming the coating bead at an edge of the substrate, in order to coat the substrate in a single pass, can be problematic.
Likewise, the initially formed coating bead may only correspond to a reduced portion of the extrusion orifice, causing coating material to accumulate, such as in drips or other accumulations of material. These accumulations of material may cling to the extrusion head and, upon coming into contact with the surface to be coated, result in areas of nonuniform thickness or other inconsistencies in the coating.
Accordingly, a preferred embodiment of the present invention is adapted to present a means for capturing material which might otherwise cause nonuniform coating of a surface, such as a coating bead forming surface upon which the coating bead may be formed and/or substantially reach a steady state prior to coming into contact with the surface of the substrate to be coated. The coating bead forming surface may present a surface, which combined with that of the substrate surface to be coated, substantially corresponds to the extrusion head at all positions throughout the relative motion of the extrusion head and the substrate surface.
Alternatively, the coating bead forming surface may present a surface, which combined with that of the substrate surface to be coated, initially corresponds to the extrusion head at positions of the relative motion of the extrusion head and the substrate surface. However, upon establishing a coating bead sufficiently wide to coat the widest portion of the substrate as traversed by the extrusion head, the coating bead forming surface may be discontinued.
Another alternative embodiment of the present invention utilizes a coating bead forming surface presenting a sufficient area upon which to establish a portion of a coating bead and that portion of the bead reaching a steady state, i.e., drawing the material accumulated in the areas of the extrusion orifice initially corresponding to neither the bead forming surface nor the substrate surface and providing sufficient surface upon which to establish a consistent flow of coating material from the extrusion orifice, prior to the relative motion of the extrusion head and the substrate surface causing that portion of the coating bead coming into contact with the substrate surface. This preferred embodiment allows a minimum amount of coating bead forming surface to be utilized while still providing a uniform coating on the substrate when using linear extrusion to coat a substrate surface which does not correspond to the extrusion head at all positions.
Additionally, or alternatively, the use of a gradient flow extrusion head, whether a variable or fixed gradient, may be utilized according to a preferred embodiment of the present invention. For example, an extrusion head having a fixed gradient of material delivery across the extrusion orifice corresponding to the surface area to be coated by each portion of the head may be utilized. Such a head may utilize a gradient shim disposed between the extrusion die halves in order to establish a graduated extrusion orifice as desired.
Similarly, an extrusion head adapted to adjust the flow of coating material in connection with the surface area to be coated corresponding to the extrusion orifice may be utilized. For example, by flexing of the extrusion head to increase/decrease the gap between the extrusion head and surface to be coated to discourage/encourage the forming of a coating bead or extrusion of material at particular points in the relative motion of the extrusion head may be contained. Likewise, controlled delivery of coating material to portions of the extrusion orifice, such a through fluid flow restrictions in the manifold of the extrusion head, may be used to control delivery of the coating material to correspond to the surface to be coated throughout the relative motion of the extrusion head.
As the thickness of the coating formed by the extrusion of the present invention is to be precisely controlled and the coating bead is preferably to be substantially at a steady state when coming into contact with the substrate surface to be coated, the bead forming surface is preferably cleaned of coating material prior to its use in subsequent extrusion coating of a substrate. However, due to the very thin coating applied by the present invention, it is anticipated that cleaning of the bead forming surface may be performed only after a number of cycles determined to result in undesired coating characteristics. Accordingly, in either case, the coating material will not build up to a point sufficient to cause irregularities in the transition region between the bead forming surface and the substrate surface.
Cleaning of the bead forming surface may include any of a number techniques or a combination thereof. For example, the bead forming surface may be coated with a non-wetting substance, such as TEFLON, in order to be more easily cleaned. However, it should be appreciated that such a non-wetting coating must not be too non-wetting as it is desired that a coating bead be formed thereon. Additionally, or alternatively, the bead forming surface may be adapted to be suitable for automated cleaning such as by spinning and/or immersion in cleaning solution. Likewise, cleaning techniques such as solvent spray/stream clean, heated solvent spray/stream clean, application of a cleaning solvent followed by a rinsing or neutralizing solvent, mechanical action, such as a brush, with or without the aid of a solvent, use of ultrasonic devices, such as an ultrasonic nozzle for application of solvent, freezing the coating material, such as by introduction of liquid CO2, or other low temperature technique for simplified removal of the material, and/or the use of disposable or cleanable shims, or xe2x80x9ctear-off,xe2x80x9d that provide the bead forming surface to be cleaned.
A technical advantage of the present invention is that a system and method for providing a uniform coating of a desired thickness on a substrate, including substrates of various shapes and sizes is provided. A further technical advantage is that the present invention provides for the efficient use of coating materials deposited thereby. A yet further technical advantage of the present invention is that uniform coatings of a material initially in a very viscous fluid state may be accomplished.
A still further technical advantage of the present invention is that coating of surfaces of the substrate which are not desired to be coated is minimized.
Additionally, a technical advantage of the present invention is that simple and efficient cleanup techniques may be used to remove any excess coating material thereby reducing the time and effort required in cleaning such excess material as well as reducing the amounts of solvents required in such cleanup.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.